1. Technical Field
The present invention relates to a paint stripping composition used in a process where it is applied to a substrate coated with fully or partially cured paint. The composition of the present invention has particular utility for use in stripping paint and will be described in connection with such utility, although other utilities, for example, stripping organic polymer coatings, also are contemplated. The present invention is also directed to a method of employing the stripping composition.
2. Brief Description of the Background Art
A paint or varnish remover should ideally remove all traces of the coating quickly and with a minimum of labor, and it should leave the substrate unharmed and suitable for reprocessing or refinishing. A number of design criteria have been identified for a modern stripper: stripping ability, lack of corrosiveness to substrates, freedom from galvanic attack at the juncture of dissimilar materials, freedom from noxious and toxic chemicals, good shelf life, high viscosity, where needed, sealable internally or with a water cap to prevent rapid evaporation, and ability to leave a readily recoatable surface. See The Encyclopedia of Chemical Technology, Third Edition, Volume 16, 762-768 (1981).
Methylene chloride paint strippers have long been the industry standard for paint stripper performance. It has been established that methylene chloride is effective for quickly softening most types of paints. It has been suggested that the effectiveness of methylene chloride results from its smaller molecular size. Its low molecular weight enables it to penetrate rapidly into a coating, and its intermediate solvency enables the coating not to be dissolved so that redisposition on the substrate is avoided. Methylene chloride removers have also been modified to increase stripping power for special purposes. These modifying chemicals include amines, alkalies and organic acids. See U.S. Pat. No. 3,538,007 to Cooper et. al. Other reported uses of methylene chloride include solvent degreasing, plastics processing, blowing agent in foams, solvent extraction, a solvent for cellulose acetate, and as an aerosol propellant.
The safe use of methylene chloride has been questioned by Federal agencies including the Food and Drug Administration (FDA). As a chlorinated solvent, methylene chloride has now been closely regulated for environmental protection. Recently, it was discovered that methylene chloride can cause tumors in rats and mice. The LD.sub.50 orally in rats (young adults) has been found to be 1.6 ml/kg. Furthermore, the threshold limiting value (TLV) has been set at 100 ppm in air. See Kimura, et. al., Toxicol. Appl. Pharmacol. 19, 699 (1971). Since these discoveries, the wide use of the solvent by industry and consumers has come under close scrutiny by federal regulatory agencies. See, U.S. Pat. No. 4,749,510 to Nelson. Accordingly, it is now highly desirable to avoid the use of methylene chloride in paint stripping formulations.
The search for an alternative product to methylene chloride has focused attention on pyrrolidone type compounds, more specifically, N-methyl-2-pyrrolidone (NMP). NMP is less toxic than many competitive aprotic solvents and does not appear to be a sensitizing agent. NMP, a dipolar aprotic solvent, has a high dielectric constant and cannot donate protons for hydrogen bonding. Consequently, many of its applications involve its strong and frequently selective solvating power. NMP is commercially available (e.g., from BASF Wyandotte or GAF) and used industrially as a processing aid for polymeric resins, in petroleum processing (e.g., the extraction of aromatics from lube oil), for the production of electronic circuitry, and as a general industrial cleaning ingredient. NMP is a colorless liquid with a mild amine odor, is miscible with water as well as various organic solvents. See the Encyclopedia of Chemical Technology, Third Edition, Vol. 19, 514-520 (1981).
Although NMP was recognized as an early candidate for replacing chlorinated solvent type paint strippers, it was soon discovered that a formulation based entirely on NMP had a number of disadvantages.
For example, NMP has low viscosity and low adhesive properties which work against its efficiency to remove dried coats of paint on a vertical surface. In U.S. Pat. No. 4,764,222 to Colegrove, a NMP composition is reported where a viscosifying agent (rhamsan gum) is added to the NMP to improve the ability of the composition to cling to a painted surface when inclined in a vertical position. An effort to optimize or target the paint removing efficiency and water solubility of NMP, while at the same time controlling and minimizing any potential toxicity and volatility, has led to a variety of specific formulations combining NMP with other organic and inorganic compounds. In U.S. Pat. No. 4,276,186 to Bakos et. al , cleaning compositions containing at least 50% by weight of NMP and at least about 5% by weight of a water miscible alkanolamine and about 0-35% by weight of a hydrocarbon solvent were described as especially suitable for removing solder flux from a ceramic substrate. In U.S. Pat. No. 4,664,721 to Valasek, non-aqueous compositions of NMP, butyl cellosolve and cyclohexanone and a surfactant are disclosed which degraded or solubilized broad classes of printing ink residues on printing screens. In U.S. Pat. No. 4,812,255 to Suwala, an aliphatic organic acid, an alkylene glycol ether and NMP are combined and reported effective as an overall composition for paint removal. In U.S. Pat. No. 4,732,695 to Francisco, a paint stripper and coatings remover composition consisted of three essential components: NMP, a mixture of aromatic hydrocarbons and benzyl alcohol. Finally, in U.S. Pat. No. 4,749,510 to Nelson, there is described a paint stripping composition comprising about 20-90% by weight NMP, 30-70% by weight aromatic hydrocarbon solvent, and about 1-15% by weight of either formic, oxalic, acetic, citric, gluconic or glutamic acid. According to another aspect of Nelson, organic amines such as diethanol amine could replace the various organic acids.
The combination of NMP with a variety of operative organic compounds represents what has been a long-standing effort to maximize the stripping ability of this alternative paint and coatings remover. While some of the presently disclosed paint strippers have relied upon formulations and proportions of active organic compounds in combination with NMP, none have recognized the novel and enhanced solvating power of the present invention which utilizes components that are commercially widely available, are less expensive than NMP, remove certain paints, coatings, inks or crayon at the same or better rate than NMP alone, can replace the majority of NMP in a given formulation, and are biodegradable, non-flammable and non-toxic.
It is accordingly an object of the present invention to provide an effective non-methylene chloride paint stripper composition that can be readily mixed and formulated to produce a stripper with the aforementioned solvating capability.
Another object is the provision of a stripper of the above type which can be applied by any conventional method, including wetting, wiping, spraying, brushing, dipping, troweling or otherwise.
Yet another object is to provide an effective composition with the aforementioned properties for cleaning urethane, polyester and epoxy compounds off of tools, molds, etc., particularly before the cure is complete.
A still further object is to provide a stripper composition of the above type which emulsifies with and can be completely removed by water.